It is well documented that chronic exposure to cocaine leads to structural changes within dendritic spines. However, the mechanisms by which these changes occur are largely unknown. Recent findings indicate that chronic exposure to cocaine leads to an increase in actin cycling between monomeric globular G-actin and polymerized filamentous F-actin, as well as a change in the structural responsiveness of dendritic spines to an acute cocaine injection. These changes are correlated with changes in the levels of a number of proteins associated with the postsynaptic structure and regulation of actin cycling, in particular Arp3. Arp2/3 proteins serve as templates for new actin filaments, and are components of a multi-protein complex important for actin polymerization. Arp3 protein levels are transiently but strikingly upregulated (>300%) in a postsynaptic density fraction following a cocaine challenge after chronic cocaine treatment and withdrawal. Thus, regulation of Arp levels and activity may provide an important means by which protein dynamics regulate changes in spine architecture in response to experience. The experiments proposed herein will use the reinstatement model of cocaine-seeking to investigate the relationship between cytoskeletal protein dynamics and the development of long-lasting cellular changes which underscore addiction. In particular, I will seek to identify the molecular mechanism(s) that underlie the structural plasticity associated with chronic cocaine use. As the Arp2/3 complex is a critical component of the polymerization of actin filaments, I will take advantage of the identified upregulation of Arp3 in response to cocaine as a starting point at which to investigate these mechanisms. The Arp2/3 actin nucleation core is activated by the WASP/WAVE family of five proteins (WASP, N-WASP, WAVE-1, WAVE-2, and WAVE-3). A number of findings indicate that activation of Arp2/3 by N-WASP or WAVE-1 may underlie actin cycling in response to cocaine. Thus, I will address the hypothesis that the Arp2/3 activation by WASP proteins is engaged in response to cocaine, and that activation of the Arp2/3 nucleation core is a critical step in the structural plasticity of dendritic spines and cocaine-seeking induced by cocaine reinstatement. I will also investigate the possibility that N-acetylcysteine (NAC), a candidate pharmacotherapy for drug seeking, may prevent the morphological changes observed in spines, as well as the changes observed in expression of Arp3 and other markers of actin cycling. The data from this proposal will explore actin-related mechanisms that underlie the cocaine-induced neuroplasticity that contributes to relapse in cocaine addicts.